DEVELOPMENT OF CONNECTIVE-TISSUE FIBEKS. 53 



OBSERVATIONS IN DETAIL. 



The growth from a piece of chick embryo of 6 to 8 days' incubation is usually 

 in the form of a membrane closely attached to the cover-slip, and is composed of 

 large, flat cells, either connected by numerous cytoplasmic processes (plate 1, fig. 1) 

 or else crowded together so that the delicate processes from cell to cell are lost and 

 a more definite cell-wall appears (plate 2, fig. 10). The growth from older chick 

 embryos may also sometimes have the appearance of a membrane, especially where 

 the cells are spread out in a thin layer along the cover-slip. When such a growth is 

 treated with silver-nitrate stain the membrane becomes marked with more or less 

 definite cell-walls, according to the amount of crowding of the cells (plate 2, fig. 10). 

 Such a membrane has been described by Clark (1914), where the connective tissue 

 is stimulated to grow out over a very smooth surface, which Clark interprets as 

 showing that under certain conditions the connective-tissue cells may become 

 transformed into endothelium. While the pattern which appears with the silver- 

 nitrate stain is in many ways characteristic of endothelium, still growths from 

 older chick embryos (8 to 10 days) in these tissue cultures exhibited the charac- 

 teristic activities of connective-tissue cells, and in some cases fibrils were formed 

 within the cytoplasm of the cells (plate 2, fig. 10). 



The growth from an 8 to 10 day chick embryo usually has the appearance of 

 a reticulum of cells (plate 1, figs. 7 and 8). Some of these cells are of the large, flat, 

 stellate type, having processes on all sides, in which may develop bundles of fibrils, 

 which pass in more than one direction through the cells (plate 1, figs. 7, 8, and plate 2, 

 fig. 10); others are cone-shaped i. e., while the cell-body may have several short 

 processes, most of the cytoplasm is drawn out into one long process (plate 1, fig. 2, and 

 plate 2, fig. 4). Both the granular and the clear cytoplasm is continued out into 

 the one long process, which practically always extends in the direction from which 

 the cell has migrated, and although in many cases it continues back as a delicate 

 thread, passing as many as twelve or more cells, it has always a protoplasmic end, 

 either free or closely attached to another cell. These long processes usually contain 

 mitochondria and other granules scattered along their length, and never in any case 

 have they been observed to change into connective-tissue fibers. In many " film prep- 

 arations " of the subcutaneous tissue studied while alive, such long, delicate processes 

 have been observed to extend along the side or through the middle of a bundle of 

 fibrils. This is probably due to the fact that, through some stress, the cell has 

 been drawn out into this shape, either from migration or manipulation, and the 

 fibrils are those which were originally in the exoplasm of the cell. 



During the beginning migration (1 hour after explantation) of the cells in 

 the explants from older chick embryos (10 to 15 days), when certain of the cells 

 first begin to migrate it is seen that they are drawn out into exceedingly long and 

 delicate processes which ramify in all directions, as though their cytoplasm had 

 extended a great length along the fibers of the subcutaneous tissue (plate 1 , fig. 3) . 

 As the cell continues to migrate towards the periphery of the explanted piece or out 

 into the culture medium, these long processes are drawn into the cell, until finally 



